Net phone firm Skype says its service is up and running again after three days of "unprecedented" disruption.

The problems with the service began on 16 August and stopped millions of people logging in and making calls.

Skype said the ongoing disruption was caused by an unexpected interaction between its servers and users' PCs.

Despite Skype's claim to have fixed the problems some subscribers reported that they had trouble making calls throughout the weekend.

Flash flood

In a statement posted on its website, Skype said the widespread outage began after subscribers' computers around the world re-started following a security software upgrade issued by Microsoft.

The knock-on effect was that an unusually high number of people tried to log on to the system at once and the Skype network could not cope.

To make matters worse, the scale of the failure exposed a previously unknown bug in the algorithm that should have helped the Skype network recover quickly.

The statement said: "as a result of this disruption, Skype was unavailable to the majority of its users for approximately two days."

It added: "...we'd like to apologise and thank you. Precisely in that order."

The company categorically denied that any malicious elements were behind the prolonged disruption.

Exact numbers of Skype users are hard to come by but the company claims that its software has been downloaded about 200 million times and it is thought about 50 million people regularly use it.

Sites and blogs that watch Skype reported problems in many different countries including the US, Canada, Brazil, Germany and Finland.

Although Skype said the service was back to normal for the majority of its users, over the weekend of 18-19 August many subscribers said they were still having problems.

Some said calls cut out unexpectedly and others struggled to get it working.

Many bloggers inconvenienced by the outage wondered if Skype was planning to refund them for all the calls they had to re-direct to other, usually more expensive, phone numbers during the period of disruption.